The present invention relates generally to producing pattern defect inspection systems for wafers, masks, and reticles. More particularly, the present invention relates to scanning electron microscopes and their use in detecting defects in semiconductors.
Scanning electron microscope systems are conventionally used in semiconductor wafer and reticle inspections. In a conventional application, a beam of electrons is scanned over a sample (e.g., a semiconductor wafer). Multiple raster scans are typically performed over a small area of the sample. The beam of electrons either interact with the sample and cause an emission of secondary electrons or bounce off the sample as backscattered electrons. The secondary electrons and/or backscattered electrons are then detected by a detector that is coupled with a computer system. The computer system generates an image that is stored and/or displayed on the computer system. The signal or image from a pattern on the inspected sample is then compared to a reference signal or image corresponding to the same pattern at another location, another wafer, or stored design data. The defects are identified from the differential signal.
The SEM approach provides superior resolution to optical inspection techniques due to the significantly shorter wavelengths used. However, the conventional SEM single electron beam approach provides a low throughput due to several physical limitations of the system.
The use of the electron beam for inspection permits high resolutions to be obtained due to the small sizes of the beam area focused on the wafer (xe2x80x9cspot sizexe2x80x9d). The high resolutions obtainable come at the expense of the throughput. For example, a 300 mm diameter wafer will require an inordinately long inspection period when a single electron beam inspection technique is used. As feature sizes used in semiconductor devices continue to shrink, the smaller spot size of the single electron beam, for example, as small as 50 mn or less, will aggravate the throughput problems. Presently, sequential scanning using a single electron beam combines mechanical movement of a stage holding the sample in a linear direction and an electrical scan of the beam. Achieving significant improvements using the same sequential scanning methods requires unrealistic speeds for the stage movement or the electrical scan.
Moreover, electron beam currents are limited by space charge effects from negatively charged electrons directed to an area of the inspection sample. A faster inspection scan using an electron beam would require a higher electron beam current. Thus, further reductions in the time required to scan a single pixel are limited by the space charge effect.
Multiple beam inspection systems have been proposed as a solution to many of these problems. However, several technical hurdles have prevented their implementation. An array of multiple columns having individual electron beams requires a large number of controls for each of the beams and wiring for each control. One criterion used in evaluating electron lenses is the spherical aberration of the lens or focusing device. Spherical aberrations are defined as the tendency of the outer zones of the lens to focus more strongly than the inner zone, thus resulting in a diffused focus area rather than a single point of focus. While micro lenses available for use in miniaturized columns in some applications have a small spherical aberration coefficient, it still limits the available current for each column and thus limits the throughput.
What is needed is an apparatus that provides an increased throughput for electron beam scanning while providing a high resolution inspection signal.
To achieve the foregoing, and in accordance with the purpose of the present invention, a multiple electron beam inspection system using uniform focus and deflection fields is described. The method for inspecting samples uses a multiple beam electron system having a uniform magnetic focusing field. Deflection of the incident electron beams is uniformly produced by deflector plates generating a uniform electrostatic deflection force. Thermal field emission sources generate incident electron beams towards at least two portions of the sample.
In one aspect, a multiple beam electron inspection system generates a first and second incident electron beam from a first and second thermal field emission source. Two polepieces generate a uniform magnetic field to focus the first and second incident electron beams on a sample. A deflector directs the first and second incident electron beams towards the sample and directs a first and second detection electron beam from the sample to a first and second detector.
In another aspect, the deflector is configured to generate a uniform electrostatic deflection field and comprises at least two plates positioned on opposite sides of the incident electron beams.
In another aspect, the first and second incident electron beams are deflected to nominal positions on the detectors using a DC bias voltage applied to the deflector. Electronic scanning is performed by applying an AC voltage to the deflector.
In yet another aspect an electrode is combined with the inspection system to generate a retarding field. The retarding field decelerates the incident electron beams but accelerates the detection electron beams.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures, which illustrate by way of example the principles of the invention.